The present disclosure relates to electronic circuits and methods, and in particular, to buffer circuits and methods.
Buffer circuits are used widely in a variety of electronic circuit applications. Buffer circuits are often used to allow different functional circuits to work together to perform signal processing tasks. For example, FIG. 1A illustrates an application of a buffer circuit 102. In this example, a signal Vin is amplified by an amplifier 101. Amplifier may be a high gain amplifier that increases a voltage amplitude of Vin, but has a low output current and/or a constrained output voltage range. It may be desirable to provide the amplified version of Vin to another processing circuit 103, referred to here as a load circuit. Load circuit 103 may require a larger input current or voltage range for proper operation than amplifier 101 is capable of producing. Accordingly, in this example, a buffer circuit 103 may be receive the amplified version of Vin and generate a signal with enough current and across a wide enough voltage range to meet the requirements of load circuit 103. Different buffer circuits may increase current, voltage, or both, for example, to allow different functional circuits to process signals in a signal path.
One example use of a buffer circuit is in a low drop out (LDO) regulator. A low drop out (LDO) regulator is a voltage regulator that can operate with a very small input-output differential voltage. FIG. 1B shows an example LDO. The LDO includes a pass transistor 100, an error amplifier 104, buffer circuit 110, a voltage divider (e.g., resistors R1 and R2), and an external load 106. Resistors R1 and R2 divide output voltage Vout to produce a divided output voltage Vo_div. Vo_div is coupled to one input of error amplifier 104. A second input of error amplifier 104 receives a reference voltage, Vref. Error amplifier 104 compares the divided output voltage Vo_div to reference voltage Vref and produces an error signal that may be coupled to pass transistor 100. If Vout increases and causes the divided output voltage to increase above the reference voltage, the error signal drives the pass transistor to reduce current into the load and reduce Vout. If Vout decreases and causes the divided output voltage to fall below the reference voltage, the error signal drives the pass transistor to increase current into the load and increase Vout. Accordingly, the LDO operates to maintain a constant output voltage Vout over changing current demands of the load 106.
In many application it would be desirable to have a buffer circuit with a wide output range that can drive an input of a subsequent circuit stage to a low voltage for a given range of voltage inputs, for example. For instance, referring to FIG. 1B, a buffer circuit 110 may be used between error amplifier 104 and pass transistor 100 to increase the drive strength to the pass transistor. However, if buffer circuit 110 has a constrained output voltage range, the output of the buffer circuit may not be able to drive the input of the pass transistor across a range of voltages for optimum performance. In particular, large currents into load 106 may require a buffer circuit in an LDO application to drive the input of the pass transistor close to ground. Accordingly, it would be advantageous to have wide output range buffer circuits and methods with improved output ranges in LDO and many other applications.